1. Field of the Invention
The present invention relates to a method for manufacturing structure, a medium for forming structure used for the method for manufacturing structure, structures obtained by the method for manufacturing structure, and an optical recording medium which performs recording and reproducing by the structures (concavity and convexity patterns) and a method for reproducing the optical recording medium.
2. Description of the Related Art
In recent years, reproduction-only optical recording media (herein after, may be referred to as “ROM disc”) composed of microscopic structures are widely spread for use with DVD-ROM being mainly focused. And the development of blue-laser, high-density ROM discs is being rushed.
The ROM disc is for recorded information by relief patterns of concavity and convexity and generally manufactured through a complicated process including master preparation, stamper preparation and replication steps.
A master is prepared by the procedures of (1) photoresist exposure by laser beams or electron beams, (2) pattern forming by resist development and (3) substrate etching using a resist as mask in the master preparation step.
A stamper is prepared by the procedures of (1) nickel (Ni) plating on the master and (2) Ni separation in the stamper preparation step.
A predetermined concave-convex pattern is transferred to a resin material using the stamper as mold in the replication step.
Further, a test recording (authoring) is performed for the purpose of confirming and adjusting recording condition, compression efficiency and coding, etc. in manufacturing process of ROM disc. There is a limitation on the use of ROM disc which has been manufactured through all steps of the manufacturing process of ROM disc for authoring in terms of cost. Therefore, a recording medium having a recording layer which contains phase-change materials or organic dyes is used as a medium for test recording (herein after, may be referred to as “medium for authoring”) in order to easily confirm authoring, etc. This kind of medium for authoring is disclosed in Japanese Patent Application Laid-Open (JP-A) Nos. 11-328738 and 2001-126255, for example.
Meanwhile, the optical recording media which perform recording of information by existing relief patterns of concavity and convexity have a problem of difficulty in microscopic concavity and convexity transferring with higher densities. To settle the problem, a mastering technique by electron-beam printing of high density is proposed in JP-A Nos. 2001-344833 and 2003-051437.
However, resist sensitivity relative to the electron beam is unsatisfactory in the electron-beam printing and since it is a process in vaccum, degradation of throughput is inevitable. Furthermore, an electron-beam printing apparatus is very expensive and a huge initial investment is needed. Moreover, throughput is degraded because of difficulty in maintenance and more running cost compared to the laser-beam exposure. Therefore, a problem arises such that the process cost rises sharply because of increase in initial investment and running cost, etc.
As a measure to settle the problem of rising process cost with micronization, a method for forming microscopic concave-convex patterns by laser beam has been developed, for example. This is a method to form patterns by disposing a thermally transformable layer, transforming a region which is smaller than the beam diameter, and removing untransformed region by etching.
For example, a method for forming concave-convex patterns (structures) by crystallizing a phase-change film such as GeSn, etc. by laser beam irradiation, and removing amorphous region by etching is proposed in JP-A No. 9-115190. Moreover, a method in which a thin auxiliary film is formed first and a groove is formed on the film once by etching and then a phase-change film formed afterward is processed by etching again is disclosed. Furthermore, a method for forming concave-convex patterns (structures) by crystallizing chalcogen compound such as GeSbTeSn by laser beam irradiation and removing amorphous region by etching is disclosed in JP-A No. 10-97738.
However, in order to form structures with appropriate uniformity on a substrate of large area such as optical discs, it is required to have large etching rate difference (etching selectivity) between regions forming structures and other regions. In the case of phase-change material, etching selectivity between crystalline state and noncrystal state (amorphous state) is small. And also, intermediate state between crystalline state and amorphous state may be formed. Therefore, it is difficult to uniformly form microscopic structures on a medium of large area by the methods disclosed in JP-A Nos. 9-115190 and 10-97738. Moreover, the manufacturing method which requires etching step twice as disclosed in JP-A No. 9-115190 has a drawback of causing a process cost rise.
Further, a method for forming structures by forming a reaction region (reaction region becomes an alloy of two metallic materials) through interdiffusion of two metallic materials by irradiating a laser beam to a thermosensitive material which is composed of stacked two metallic materials such as Al/Cu and by removing unreacted region by etching is disclosed in JP-A Nos. 2001-250279 and 2001-250280.
Moreover, a method for forming structures by forming a reaction region through interdiffusion of two materials by irradiating a laser beam to a stacked composition containing two inorganic materials such as Au/Sn and removing unreacted region by etching is disclosed in JP-A No. 2003-145941.
However, it is difficult to uniformly form microscopic structures on a medium of large area by these methods because thickness distribution of interdiffusing two materials directly becomes a composition distribution of regions forming structures, and the etching rate varies if the composition is different.
Further, a method in which a laser beam is irradiated to a stacked composition of a light absorption, heat transfer layer such as GeSbTe, etc. and a thermosensitive layer made up of chemically-amplified resist used for photolithography to transform the thermosensitive layer and untransformed region is removed by etching to form structures is proposed in JP-A No. 2002-365806.
However, materials forming the structures in JP-A No. 2002-365806 are light absorption materials and the method in which the light absorption material is used as a structure-forming material is not suitable for forming structures of high aspect ratio (height of pattern/size of structure). In other words, in the case of forming structures of high aspect ratio, a layer forming structures is needed to be thick, however, thick layer prevent micronization because heat is spread through the layer.
Therefore, a method for manufacturing structure which can form microscopic structures inexpensively by a simple process without using photolithography, and an optical recording medium having the structures uniformly on the medium of large area have not been provided and their prompt provision is desired in current condition.